This research is an extension of previous work using polyoma mutant viruses and viral sequences with recombinant DNA probes to study the gene expression requirements of teratocarcinoma cells. Teratocarcinoma cells consist of a stem cell, embryonal carcinomas (EC), which can be grown as a cell line under cell culture conditions, and then depending upon the cell line used, can be induced to differentiate to other cell types. We are using the viral and recombinant DNA probes to examine what DNA sequences are necessary for expression of genes in these different cell types and to try to select for cellular sequences which might play a role in cell type specific gene expression. This work makes use of various gene transfer techniques, manipulation of small restriction fragments containing viral enhancement sequences, oligonucleotide synthesis to reconstruct regulatory sequences, and competition transfection experiments to examine whether regulatory sequences are competing for cellular factors which play a role in enhancing gene expression in teratocarcinoma cells. (G)